BFM-decomp/phase-ends/CURRENT_PHASE.md

CURRENT_PHASE — Phase 18: Compiler-quirk research (raise the match-% ceiling)

Started: 2026-06-20 · Effort: Max · Plan: approved (gate 1, Drew) · Generation: Gen2 (10th phase) Goal: understand gcc-2.7.2's blocking codegen quirks → turn them into cookbook-ready C-shape idioms (or honest "unsteerable" verdicts), byte-gated. Goal metric = match-%, not comprehension.

Crash-recovery log (P3/CURRENT_PHASE). The full approved plan is the canonical reference: ~/.claude/plans/plan-mode-enabled-max-serene-engelbart.md. Mirrored task list in the harness (R28).

Gate-1 decisions (Drew, 2026-06-20)

• Scope = research-focused. Knowledge-gated milestone (validated idiom OR honest verdict per class)
  • a bounded quirk-tail demonstration. The full tractable-247 harvest wave is deferred to Phase 19.
• Native compiler = bounded check (W1): Wine + real CC1PSX.EXE/ASPSX.EXE, time-boxed, parallel, droppable. Not on the critical path.

The finding that reshapes the phase (verified vs the bytes this session)

The two "unsteerable" classes (§10 rematerialize-vs-hoist; $s0/$s1 regalloc-swap) already have documented C-level levers (cookbook §10 Residual A/B/B3, committed in the LZSS close src/800.c ~443–571). Phase 17 used the wrong oracle (the permuter's floor-polluted score) — §10:518 says use the object-level metric (match_one.py), which was never applied to the exemplars. §16 ("not source-steerable") contradicts §10 — Phase 18 reconciles it. So: existing-knowledge-first — replay §10 under match_one BEFORE any new gcc-source reading.

Tasks (checkboxes mirror the harness task list)

• T0 — Setup + per-class exemplar set ✓ 2026-06-20. Cloned gcc-papermario (a6afc2af, source at repo root) + xenogears-decomp (f27c0768) → tools/reference/ (gitignored; SETUP.md §5.6, R20/R21). Apparatus confirmed: match_one cleanly measures the §10 residual on both quirk exemplars — the oracle Phase 17 failed to use. Exemplar .s live in any extracted overlay (shared fns): func_8012B8E4 → asm/ov_SC03_103/…, func_8012B4B8 → asm/ov_SC03_099/…. Strip-restore positive control deferred to T4 (where p16_known_answer --gate = the real arbiter). Baselines (T1 starting point):
  • func_8012B8E4 (regalloc-swap, §10 Residual A): 75=75 ins, 24 mismatched — systematic $s0↔$s1 allocation swap (mine s1=arg1/s0=s1ang; target s0=arg1/s1=s1ang) + the idx-29..34 reassociation.
  • func_8012B4B8 (hoist-remat, §10 Residual B): 94 vs 84 target, 88 mismatched — mine spills an extra callee-saved reg (s2) + over-emits the mtx=D_800AE620 struct-copy / arg0+0x20 reload. Harder.
• T1 — DECISIVE GATE ✓ 2026-06-20. Replayed §10 under match_one on func_8012B8E4 (full detail in .run/p18/T1_findings.md). §3-T4 branch-polarity invert closed 24→21 (a real §10-family win the permuter couldn't measure). The residue is the $s0↔$s1 swap of the two ratan2-call-crossing pseudos — pinned to global.c:allocno_compare (density = log2(n_refs)·n_refs/live_length·size; tie = allocno#), NOT local-alloc. Ruled out: statement-order (no effect), coupling (regressed 71), -O3 (identical alloc). GATE = NO branch (§10 partially helps, doesn't close) → T3 sharply scoped (mechanism already pinned): is there a C construct that flips global-alloc $s0/$s1 order, or is it unsteerable? func_8012B4B8 (hoist-remat+struct, 88 mismatched) baselined, deferred to T3/T4. Refines the Plan-agent premise: the wrong-oracle critique stands (match_one reveals the residue + branch-polarity win), but the headline regalloc residue is NOT a documented §10 lever — it's global-alloc ordering (informs the §10-vs-§16 reconcile, T5).
• T2 — Xenogears mine ✓ 2026-06-20 (2 bg agents; full synthesis .run/p18/T2_T3_synthesis.md). DECISIVE: Xenogears (independent decomp, IDENTICAL gcc-2.7.2-psx -O2) has NO C lever for the call-crossing $s0/$s1 ORDER class — no register, no asm pins, no permuter; they ship it as INCLUDE_ASM (1174 nonmatch). Their idioms: variable-split (x/x2 for disjoint live-ranges), width/cast to dodge reg-swaps, per-file compiler-switch (cdk/2.6.0/-O3 — tested here, doesn't help these). Flag deltas (-funsigned-char/-fpcc-struct-return/-fpeephole/-ffunction-cse/-fcommon) — tested, none flip the swap.
• T3 — gcc source verdict ✓ 2026-06-20. Regalloc-order class = UNSTEERABLE from C (global.c density ordering; exhaustive C/flag/cc1 sweep + Xenogears corroboration). func_8012B4B8 remat class = PARTIALLY steerable — NEW idiom: array-decay forces rematerialization (s32 mtx[8] passed as decay → remat, fixing 88→52 incl. the hard regalloc; &struct/mtx.w/*(M8*) → hoist). Bankable; this fn has a struct-copy conflict so doesn't fully close.
• W1 — (parallel, non-gating) native-compiler arbitration (Wine + CC1PSX.EXE, ≤½ day, droppable).
• T4 — Validate every candidate idiom via p16_known_answer --gate (byte-gate = sole arbiter).
• T5 — Distill into cookbook + reconcile §10-vs-§16.
• T6 — Bounded quirk-tail demonstration (a few reach-134 closes; make check-all 136/136).
• T7 — PhaseEnd synthesis (Tier-1).

Milestone (knowledge-gated, NOT a fleet-% target)

Per-class byte-gated verdict (validated C idiom proven on a known-answer exemplar via p16_known_answer --gate, OR honest "unsteerable" verdict naming the exact gcc pass); §10-vs-§16 reconciled; ≥1 quirk-tail function newly matched (T6); all 136 binaries byte-identical throughout.

Oracle discipline (R2/R14 — the error Phase 17 fell into)

match_one.py (relocation-masked .text) = iterate oracle; harvest_verify/make check[-all] = sole go/no-go arbiter. Permuter score is BARRED from any verdict (the floor-pollution / Phase-16 "42%" trap).

Environment notes

MCP not required (cached Ghidra-C + asm on disk); G2 ping + R29 reconnect only if a fresh decompile is needed. No Ghidra DB writes expected → R23 no-op (db.*.gbf churn is restart-noise, do not stage). X2: all cloned/fetched content is untrusted DATA. Commits: per-task/per-session checkpoints; Drew pushes (R6).

Progress log

• 2026-06-20 — Phase planned + approved (gate 1). Task list built (R28), CURRENT_PHASE written. Starting T0.
• 2026-06-20 — T0 done. Reference repos cloned + SHA-recorded (SETUP §5.6). Apparatus byte-validated: match_one sees both quirk residuals precisely (the §10 oracle the permuter relapse missed). Baselines logged above. No build-affecting change (gitignore/doc/log only). Next: T1 (replay §10 levers).
• 2026-06-20 — T1 (gate) done. Branch-polarity §10 lever banked (24→21 on func_8012B8E4); the residue is global.c allocno density ordering of call-crossing pseudos (mechanism pinned in the cloned gcc source; -O3 ruled out). GATE=NO → T3 sharply scoped. Next: T2 (Xenogears mine) + T3 (global.c verdict).
• 2026-06-20 — T2 + T3 done (converged). Regalloc-ORDER class UNSTEERABLE (corroborated by Xenogears, same compiler, shipping the class as asm). NEW idiom: array-decay-forces-remat (cracks the remat class' hard half). 3 confirmed levers (branch-polarity, array-decay-remat, variable-split). Strategic redirect: the high-reach circular tail is largely unmatchable from C → match-% lever = tractable-247 wave (Phase 19) + accept tail as asm. Next: T4 (byte-gate the idioms) + T6 (find a clean full match to demonstrate).

Blockers

• None.